1. Field of the Invention
This invention relates to an apparatus for detecting a level deviation or defect of a signal such as a reproduced signal read out of an information recording medium.
2. Description of the Prior Art
A variety of types of apparatus have been developed lately for recording video or audio information on an optical disk and then reproducing the information. They have a contact-free reproduction ability and ,a high density recording capability. Signals are recorded at high density in the recording layer of the optical disk. The recording layer is protected from damage due to oxidation or mechanical shock by using a protecting layer of a light-transmitting resin material or the like. The reproducing head is indispensably controlled by a servo mechanism for focusing and tracking.
A disk having a resin protecting layer could be easily scratched or injured on its surface, causing a flow, if it is not handled carefully. Such a flaw of the disk is fatal to the entire system. For example, the servo mechanism of the reproducing head could kick off to an adjacent track due to disturbance by the flaw, or the error the reproduction head in the demodulation circuit caused by the flaw would propagate even after the head has passed through the flaw portion. To lessen these adverse effects due to the flaw, it is desireable to detect the flaw in real time and correct the disturbance of the reproduced signal.
Incidentally, in the servo mechanism, since the supply voltage is limited, the vibration resistant performance is limited. Therefore, due to an unexpected outside impact the reproducing head would be kicked off its desired track to an adjacent information track. As a countermeasure, it is necessary to detect the off-track state of the reproducing head, and return it to the original reproducing position as quickly as possible.
Referring now to the drawings, one of the conventional signal level deviation detecting apparatus will be described below.
FIG. 11 is a block diagram of a conventional signal level deviation detecting apparatus. FIGS. 12 (a)-(c) show the respective signal wave forms of A0, B0 and C0 in FIG. 11. The reproduced signal A0 is actually a sinusoidal wave, but is schematically represented as a triangular wave.
The reproduced signal A0 read out from an information recording medium 1 (hereinafter referred to as disk 1) is converted into a rectangular wave B0 by a waveform shaping circuit 4. It must be noted here that the signal recorded in the disk 1 is generally limited in the frequency band, so that the longest period of the reproduced signal A0 is controlled at a constant value. For example, in the EFM method used in a CD player, if the fundamental clock period is assumed to be 1T, the longest period is 11T.
However, if there is a flaw in the disk 1, the information recording layer of the disk 1 is concealed by this flaw, and the reproduced signal A0 will not be obtained correctly. Accordingly, the period of the rectangular wave B0 exceeds the longest period in the section of the flaw. Therefore, if the rectangular wave B0 is fed into a retriggerable monostable multivibrator 5 (RMM 5), which has an output pulse width slightly longer than the longest period, the logic of output signal C0 of RMM 5 is inverted in the section of the flaw. Thus, the section having the flaw in the disk 1 can be detected by making use of the phenomenon that the deviation of the period of the reproduced signal exceeds a specified width.
In such a constitution, however, the rotational period of the disk 1 is required to be nearly in coincidence with the rotational period at the time of recording. That is, since the longest period of the reproduced signal is in linear proportion to the rotational period of the disk 1, and if the rotational speed of the disk 1 is low, the longest period of the reproduced signal is longer than the specified value. This tendency is, particularly when the disk 1 starts rotating. Therefore, it is required that the output pulse width of RMM 5 be sufficiently long in order to avoid malfunction when the rotational speed of the disk 1 is low. As a result, after the disk 1 has reached the normal rotational speed, the detection of deviation of the reproduced signal A0 is delayed by a time corresponding to the extra portion of the pulse width. Further, the method of determining the output pulse width generally makes use of the time constant of charge and discharge determined by a capacitor and a resistor. Therefore, the circuit, if integrated on an IC chip, requires an external capacitor, so that it is disadvantageous for reducing cost or size. Even if function equivalent to the time constant is realized by using a binary counter, correct counting is difficult unless the clock for counting is synchronized in phase with the reproduced signal.